Work centers on the mechanism of action of hormones which alter fluid and electrolyte transport--the primary focus being the action of aldosterone. The physiological effects of aldosterone require a receptor-mediated rise in RNA and protein synthesis. The induced proteins are thought to work at several key sites in the cell--sites which govern rate-limiting processes for the transport of Na, K and H ions. In general, all of the suggested pathways by which aldosterone-induced proteins act, are thought to ultimately affect either the entry of Na into the cell, the provision of energy or substrate for transport, or the coupling ratio/level of functional Na-K ATPase molecules. However, most of the above thesis is predicated on studies in toad bladder. In the past, work has involved several facets of this model including characterization of both the cytoplasmic mineralocorticoid receptor and the chromatin-binding site for receptor complexes. Because continuation of these studies was hampered by lack of defined target site(s) along the mammalian nephron, I exploited the use of citrate synthase (CS), a biochemical marker of aldosterone-sensitive sites, to help identify cellular targets for aldosterone in the rabbit kidney. Those sites which showed maximal aldosterone-sensitive changes in CS activity were the cortical collecting tubule (CCT) and the medullary thick ascending limb of Henle (mTALH). In parallel studies, the PI also determined that aldosterone increased Na-K ATPase activity in the CCT. In addition, biochemical and physiological studies by the PI recently led to the clear identification of the distal colon as a target site for mineralocorticoids. Thus, the current research plan furthers our developing model of aldosterone action by: 1) examining the role of steroid and intracellular Na in the activation of NaK ATPase activity in the CCT, 2) examining the means by which aldosterone alters citrate synthase levels, and 3) continuing this analysis in rabbit distal colon, a tissue in which biochemical and physiological events can be more readily correlated.